1. Field of the Invention
The present invention relates to an image forming apparatus using xerography, such as an electrostatic copying machine, a laser printer, or the like, and in particular, to an image forming apparatus having a contact-charging/toner-cleaning blade that removes residual toner from an image bearing member, and charges the image bearing member by contacting the image bearing member.
2. Discussion of the Background
A charging device that serves as a cleaning device that removes residual toner after image transfer from a surface of a photoconductive element and a charging device that uniformly charges the photoconductive element with a conductive blade are known.
For example, a charging device that charges and cleans the photoconductive element by contacting a conductive plate-shaped member onto a surface of the photoconductive element having a surface of an amorphous silicon layer is disclosed in Japanese Laid-Open Patent Publication No. 60-147756/1985.
Further, a blade whose electric resistance is in a range of 10.sup.5 to 10.sup.9 .OMEGA. and whose contacting pressure to the photoconductive element is at least 1.5 g/mm is disclosed in Japanese Laid-Open Patent Publication No. 7-92769/1995.
In these background arts, the blade scrapes off residual toner from the photoconductive element and charges the same downstream of the rotational direction of the photoconductive element. However, generally, the cleaning method using the blade cannot perfectly remove the residual toner from the photoconductive element, and about 2 to 10 pieces of residual toner may remain on the photoconductive element and that have been rubbed with the blade. Furthermore, both of positive and negative polarities exist in the residual toner in a mixed state, and in the toner that has been passed by the blade, both of the positive and negative polarities exist.
Accordingly, in a case of a N/P (negative-positive) transfer method, since a voltage of negative polarity has been applied to the blade when the toner of positive polarity passes by the blade, the toner sticks on the surface of the blade and a sticking amount of the toner of the positive polarity gradually increases. Therefore, an image quality deteriorates due to unevenness of the density of a half tone image because of unevenness of the charge on the photoconductive element after an elapse of extended periods of time.
FIG. 12 is a schematic diagram of an image forming apparatus using an electrophotographic process. A photoconductive drum (image bearing member) 101 is provided with a photoconductive element 101a on a surface of a conductive member 101b and rotates in a direction indicated by an arrow. A charging device 102, an exposing device 103, a developing device 104, a transfer device 105, and a cleaning device 106 are disposed around the photoconductive drum 101 and each of the devices is operated as follows.
The charging device 102 charges a surface of the photoconductive element 101a to a required potential. Then, the exposing device 103 exposes the surface of the photoconductive element 101a based on image data, and forms a latent image corresponding to the image data on the surface of the photoconductive element 101a. The developing device 104 develops the latent image formed by the exposing device 103 with toner, and thereby forms a toner image on the surface of the photoconductive element 101a. Thereafter, the transfer device 105 transfers the toner image formed on the surface of the photoconductive element 101a onto the transfer medium P conveyed with a conveying device (not shown). The transfer medium P on which a toner image is transferred by the transfer device 105 is then conveyed to a fixing device (not shown). Then, the transfer medium P is disposed outside of the apparatus after the toner image is fixed, e.g. with heat.
On the other hand, the cleaning device 106 scrapes off any residual toner remaining on the surface of the photoconductive element 101a at a post-transfer, and cleans the surface of the photoconductive element 101a. Hereupon, the background image forming apparatus has been provided with the cleaning device 106 and the charging device 102 individually.
FIG. 13 shows an example of a cleaning device used in a background image forming apparatus. The cleaning device 106 is constructed with a cleaning blade 106a as a main element. The cleaning blade 106a is made of a urethane rubber. The cleaning blade 106a is formed, for example, with a thickness of 2 mm and a width of about 300 mm and is supported by a supporting member 106b. A tip end portion of the cleaning blade 106a projects from a tip end of the supporting member 106b, and the projecting length of the tip end portion is adjusted to, for example, 8 mm.
Further, a supporting angle .theta. (an inclining angle tangential to the photoconductive element 101a) of the cleaning blade 106a is 20.degree., and an amount b of the cleaning blade that cuts into the residual toner on the photoconductive element 101a is adjusted to approximately 1.2 mm. The thus constructed cleaning blade 106a scrapes off any residual toner that is stuck on the photoconductive element 101a by cutting into the residual toner.
On the other hand, a so-called contact-charging device is widely used as a charging device 102, other than a corona-charging device such as a background scorotoron. The contact-charging device charges the photoconductive element 101a by contacting a charging member to the photoconductive element 101a and applying a voltage to the charging member.
FIG. 14 shows an example of the background contact-charging device. The contact-charging device is provided with a charging member 107. The charging member 107 has an elastic layer 107a at a peripheral surface of a conductive member 107b and is formed in a cylindrical shape with a diameter of, for example, 5 mm to 20 mm, and a width of, for example, approximately 300 mm. The charging member 107 is driven by a rotation of the photoconductive element 101a contacting the charging member 107. The elastic layer 107a of the charging member is composed of a conductive member having a resistivity of about 10.sup.7 through 10.sup.9 .OMEGA. cm. Further, a surface protecting layer having a thickness of about 10 through 20 .mu.m may be formed at the surface (the surface of the elastic member layer 107a) of the charging member 107. A predetermined charging voltage is applied to the charging member 107 by a power source 108, and thereby, the photoconductive element 101a is charged. Generally, the charging voltage is about DC -1.0 kV to -1.5 kV.
As mentioned above, the background image forming apparatus that is individually provided with both the cleaning device 106 and the charging device 102 has a shortcoming that the entire apparatus has a large structure. This is because the apparatus requires both of a cleaning space and a charging space around the photoconductive drum 101. Furthermore, since the cleaning device 106 and the charging device 102 are constructed as individual members, a large number of parts is required and manufacturing costs become high. To solve such a problem, an image forming apparatus having a contact-charging/cleaning blade is proposed in, for example, Japanese Laid-Open Patent Publication No. 56-165166/1981, and Japanese Laid-Open Patent Publication No. 7-92767/1995.
FIG. 15 shows a schematic diagram of a contact-charging/cleaning blade provided in such a background image forming apparatus. The contact-charging/cleaning blade 110 is formed with approximately the same measurements and configuration as the above-mentioned cleaning blade 106. Namely, the contact-charging/cleaning blade 110 is formed, for example, with a thickness of 2 mm and a width of about 300 mm. The tip portion of the contact-charging/cleaning blade 110 projects from the tip portion of a supporting member 111, and the projecting amount a is adjusted to, for example, 8 mm. The contact-charging/cleaning blade 110 is connected to a power source 112.
Further, a supporting angle .theta. of the contact-charging/cleaning blade 110 (the inclining angle of the cleaning blade 110 tangential to the photoconductive drum 101a) is 20.degree., and the amount b of the cleaning blade 110 that cuts into the residual toner on the photoconductive element 101a is adjusted to approximately 1.2 mm. In such a construction of the contact-charging/cleaning blade 110, an edge part of the cleaning blade 110 cuts into the residual toner that sticks to the photoconductive element 101a, and scrapes off the residual toner from the photoconductive element 101a.
Furthermore, this contact-charging/cleaning blade 110 is arranged with an electric resistance of approximately 10.sup.6 to 10.sup.9 by scattering carbon and ion conductive material over the urethane rubber. In addition, a voltage is applied to the contact-charging/cleaning blade 110, similarly to the aforementioned charging member 107, and the contact-charging/cleaning blade 100 charges the photoconductive element 101a. In general, the charging voltage is about -1.0 kV DC to -1.5 kV DC.
In the image forming apparatus provided with the aforementioned background contact-charging/cleaning blade 110, there has been a possibility of the following problem occurring when the image forming apparatus is used for extended periods of time. Namely, the residual toner that sticks onto the photoconductive element 101a, even after transfer, has been considered to be removed by the edge part of the contact-charging/cleaning blade 110.
However, actually, even in a slight amount, the residual toner passes between the edge part of the contact-charging/cleaning blade 110 and the surface of the photoconductive element 101a. It is considered that a slight vibration of the charging cleaning blade 110 itself, or a vibration that is applied to the charging cleaning blade 110 at a time of starting a rotation of the photoconductive element 101a or at a time of stopping the photoconductive element 101a, may be the reason for the aforementioned problem. Furthermore, a part of the residual toner that has passed by the edge part of the contact-charging/cleaning blade 110 sticks to the surface of the blade 110 contacting the photoconductive element 101a, downstream of the edge part of the blade 110.
The reason why the toner thus sticks to the contact-charging/cleaning blade 110 is considered to be as follows. In an image forming apparatus of a so-called negative-positive development method, toner that is charged to a positive polarity (a reverse polarity to the charging polarity of the photoconductive element 101a) by an electric discharge at a time of transfer or the like exists in the residual toner on the photoconductive element 101a at post-transfer. Further, when the toner passes by the contact-charging/cleaning blade 110, a positive charged toner due to friction with the blade 110 exists. Furthermore, in a so-called positive-positive development method, the toner itself is charged to a positive polarity.
On the other hand, since a high voltage of the negative polarity is applied to the contact-charging/cleaning blade 110 when the toner passes by the blade 110, the residual toner of the positive potential sticks onto the contact-charging/cleaning blade 110 at the negative potential. Further, if the toner that is stuck to the contact-charging/cleaning blade 110 is stacked for extended periods of time, the electric discharge that occurs between the blade 110 and the photoconductive element 101a becomes uneven, and the photoconductive element 101a cannot be uniformly charged. Accordingly, a problem of an abnormal image formation occurs.
To solve such a problem, there is also proposed a device for previously charging (pre-charging) the residual toner on the photoconductive element 101a to a negative polarity, i.e., the same polarity as the polarity of the voltage to be applied to the contact-charging/cleaning blade 110, before the residual toner passes by the contact-charging/cleaning blade 110, see Japanese Laid-Open Patent Publication No. 7-325459/1995. However, in a case of adopting such a device, it is required to dispose a charging device for charging the residual toner to the negative polarity around the photoconductive drum 101, and accordingly, both of the previously discussed problems, a large sized configuration of the image forming apparatus and high costs due to a large number of the parts, cannot be avoided.